[p3lim]

Pass a function as the second parameter to oUF:SpawnNamePlates, it will be called whenever a new nameplate unit is used or when the player changes targets.
Then use UnitIsUnit('target', unit) in that function to check if they match (unit being the 3rd argument passed to the function), and proceed to show/hide your border.

Drycoded example:

oUF:SpawnNamePlate(nil, function(self, _, unit)
    if(UnitExists('target') and UnitIsUnit('target', unit)) then
        self.border:Show()
    else
        self.border:Hide()
    end
end)

Shorter version

oUF:SpawnNamePlate(nil, function(self, _, unit)
    self.border:SetShown(UnitExists('target') and UnitIsUnit('target', unit))
end)